I have a formula in cell F4 that i want to copy down to
the end of the data range. the range fluxuates daily. some
days 30 rows..some days 80 rows. so no matter what the row
range the macro automatically fills the formula down to
the bottom of the range column. Here is what I have so
far. But not working.

Range("F4").Select
ActiveCell.FormulaR1C1 = "=NETWORKDAYS(R1C5,RC[-1])"
Range("F4").Select
Range(ActiveCell, ActiveCell.End(xlDown)).AutoFill